The Arabian Shield Lithostratigraphic and Lithodemic Code

Dr. Ahmad Redaa

2026-02-02

Learning outcomes

By the end of this lecture/handout, you should be able to:

  1. Distinguish lithostratigraphic vs lithodemic units (and when to use each).
  2. Explain why the Arabian Shield requires a tailored code for mapping and nomenclature.
  3. Apply a decision workflow to classify units in arc terranes and deformed belts.
  4. Recognize how terranes and shear/fault systems constrain unit names and correlations.
  5. Use correct ranks, naming rules, and what-not-to-do pitfalls.

What is the ASLLC?

The Arabian Shield Lithostratigraphic and Lithodemic Code (ASLLC) is a set of rules and procedures for classifying, ranking, and naming geological units of the Precambrian Arabian Shield.

It provides: - standardized definitions of rock units, - clear hierarchical ranks (formation, lithodeme, suite, etc.), - consistent naming conventions, - a framework compatible with modern digital geological mapping.

Why was it established?

The ASLLC was developed to support the nationwide remapping of the Arabian Shield at 1:100,000 scale, because:

Why a stratigraphic code?

A stratigraphic code is a rulebook for:

In modern mapping programs, the code also ensures database interoperability: unit codes link maps to geochemistry, geochronology, structural data, and field observations.

Why ASLLC? The Arabian Shield mapping problem

The Arabian Shield is dominated by Neoproterozoic arc-related rocks formed during accretion and amalgamation.

Common challenges in the Shield:

The big idea: two categories of units

ASLLC deals with formal subdivision of two categories of rock units:

  1. Lithostratigraphic units (generally layered/stratiform)
  2. Lithodemic units (generally non-layered/non-stratiform)

A major additional development is a structural scheme for highly deformed rocks, and a mixed scheme using “complex” when mapping components separately is impractical.

Decision question (the most important in ASLLC)

Which scheme do I use? Ask:

  1. Are primary depositional/volcanic structures preserved?
  2. Can you establish way-up / stratigraphic order?
  3. Does the body generally obey Law of Superposition?
  4. Is deformation pervasive enough to transpose original features?
  5. Can components be mapped separately at the working scale?

Outcome:

Lithostratigraphy: what it is

Lithostratigraphy classifies stratiform volcanic and sedimentary rocks based on:

Typical properties:

Low-grade metamorphic equivalents can still be lithostratigraphic if primary structures and way-up criteria are preserved.

Lithostratigraphic hierarchy (ranks)

From highest to lowest:

Formation is the key mapping unit: identifiable, and mappable.

Formation: definition & practical mapping rules

A formation is a body of rock defined by lithology and stratigraphic position that is:

Practical points:

Arabian Shield context: “supracrustal rafts” and correlation

In arc terranes, supracrustal sequences commonly occur as isolated rafts intruded by related plutons.

ASLLC guidance:

Lithodemics: what it is

Lithodemics classifies rock bodies that are:

Key contrast:

Lithodemic hierarchy (ranks)

From highest to lowest:

Notes:

Lithodeme: definition and naming

A lithodeme is the fundamental lithodemic unit:

Naming pattern: Geographic term + lithic term
Examples: Malik Granite, Abt Schist, Dumaym Diorite.

Do not add “Lithodeme” to the name (e.g., not “Abt Lithodeme”).

Suite and Supersuite: what they mean

A Suite groups ≥2 associated lithodemes of the same class (intrusive or metamorphic).

A Supersuite groups ≥2 suites/complexes with broader natural relationship.

Important modernization:

Complex: when you are allowed to use it

A Complex is used only when mapping each component separately is impractical at the working scale.

Three accepted complex types:

  1. Complex (generic): mixed classes and/or mixed ages where components cannot be separated.
  2. Volcanic Complex: diverse, cogenetic extrusive volcanic + volcano-sedimentary assemblage (often rank-like a subgroup).
  3. Intrusive Complex: variable, coeval intrusive assemblage that cannot be separated (often rank-like a subsuite).

Key caution: - Do not use “complex” simply because rocks look complicated. - It is a cartographic and subdivision tool, not a shortcut.

Highly deformed rocks: the structural scheme (major ASLLC innovation)

When deformation transposes original fabrics and primary features such that:

ASLLC uses a structural hierarchy:

These replace older vague “structural complex” concepts.

Definitions: shear zone vs fault vs fault zone

Systems (rank above): - Shear Zone System / Fault System must include ≥2 named zones/faults and reflect a broadly coeval tectonic regime.

Transitional units in deformation zones (naming rule)

It is common for a unit to grade from undeformed → deformed → fully transposed.

Rule of thumb:

Boundaries are often somewhat arbitrary and decided by the mapping team.

Terranes and tectonostratigraphic position: why unit names may be terrane-limited

The Arabian Shield is a mosaic of originally allochthonous terranes.

Implication for naming/correlation:

This protects against false correlation of unrelated arc packages.

Formal unit requirements (what a proper unit description must include)

To establish or modify a formal unit, documentation should include:

Naming rules you must follow

Formal names are compound.

Lithostratigraphic units

Lithodemic units

Capitalization: - First letter of each word in formal unit names is capitalized.

What ASLLC explicitly does NOT do (avoid these errors)

ASLLC is not:

Terms such as arc, ophiolite, belt, pluton, ring complex are generally informal descriptors. They may appear in legends or free-text notes but are not formal database unit terms.

Why this matters for digital mapping and exploration

ASLLC is designed for a modern digital mapping workflow:

Practical value: - clearer regional synthesis, - improved reproducibility, - stronger framework for mineral and energy exploration targeting.

Worked decision example (conceptual)

Scenario: An arc volcanic sequence occurs as rafts, intruded by tonalite–granodiorite, cut by a major shear zone.

Classification:

  1. Stratiform volcanics + sediments with preserved bedding → formations (lithostratigraphy).
  2. Tonalite–granodiorite pluton with gradational internal contacts → lithodeme with compound lithic term if needed.
  3. A mixed contact zone that cannot be separated at mapping scale:
    • if multiple coeval intrusions → intrusive complex
    • if mixed classes (intrusive + supracrustal) → complex
  4. Where transposed in the shear zone:
    • identifiable protolith → same geographic name + structural descriptor + new code
    • unidentifiable protolith → structural lithodeme, within a shear zone (and potentially a shear zone system)

Arabian Shield Lithostratigraphic and Lithodemic Code (ASLLC)

Image generated by NotebookLM
Saudi Geological Survey Special Publication Report SGS-SP-2024-2 (ASLLC, 2024).